This small unnamed crater (right) in Mare Cognitum (C in Figure 91)
is unusual because it appears to be deformed by a fault that also bounds
a mare ridge. About 5 km in diameter, the crater is obviously older than
the mare materials that have buried the outer part of its ejecta blanket.
The visible part of the fault extends between the arrows and clearly transects
the western wall of the crater. It also marks the west flank of a small
mare ridge north of the crater. Viewed stereoscopically, the fault plane
can be seen to dip gently to the west, and the surface west of the fault
is lower than that on the east. The fault is, therefore, a low-angle normal
fault. The abrupt disappearance of the fault at the south rim of the crater
may seem surprising. One of several explanations is that it may lie buried
beneath a younger basalt flow that flooded the area immediately south of
the crater. Many lunar investigators, including several contributors to
this volume, have suggested a relationship between faulting and the development
of mare ridges. Although this is a very small and certainly uncommon example,
it is a convincing example of a mare ridge that is coincident with a fault
and thus lends support to this idea. -G.W.C.